Gru decides to pull of the biggest heist ever by stealing the Moon, in order to prove to everyone (especially his mother) that he is a successful supervillain. In order to do this, he must steal the Shrink Ray from the current hot-shot villain, Vector.
Noticing Vector's weakness for cookies, he adopts the three little girls who Vector buys cookies from and uses them to infiltrate Vector's lair and steal the Shrink Ray.
As he plans the Moon heist, Gru finds himself becoming increasingly attached to the orphans. This does not go unnoticed by his assistant Dr. Nefario, who arranges to have the girls sent back to the orphanage.
Gru successfully steals the Moon, but when Vector kidnaps Edith, Margo and Agnes, he trades it in return for them. He rescues the girls just in time - the Moon reverts back to it's original size; catapulting Vector into outer space.
Gru once more adopts the girls, promising never to let them go again.
Thanks Mary C!